All strong mode cleaning of dart2js.

pkg/compiler/lib/src/enqueue.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.enqueue;
 
 import 'dart:collection' show Queue;
 
 import 'common/tasks.dart' show CompilerTask;
 import 'common/work.dart' show WorkItem;
@@ skipping 584 matching lines @@
 
 class DeferredAction {
   final Entity element;
   final DeferredActionFunction action;
 
   DeferredAction(this.element, this.action);
 }
 
 /// Interface for creating work items for enqueued member entities.
 abstract class WorkItemBuilder {
-  WorkItem createWorkItem(MemberEntity entity);
+  WorkItem createWorkItem(covariant MemberEntity entity);
 }